Springs of different types are provided with the ability to dynamically change stiffness. A number of embodiments feature hollow tubing wherein stiffness change is accomplished due to the pressure change inside the tubing which affects the stresses in tubing walls. In other embodiments inside pressure change causes variability in tubing's cross-section shape and size leading to large changes in stiffness. A number of embodiments feature spring coil diameter variability achieved by a variety of means and resulting in highly substantial changes in stiffness and for some embodiments spring length variability thus providing them with actuation capability in addition to stiffness variability.